We have developed a series of monoclonal antibodies to human granulocytes, monocytes, platelets, and eosinophils. An IgG1 monoclonal antibody produced against human granulocytes reacted specifically with mature granulocytes and did not react with any other normal circulating or bone marrow cells. No reactivity was found when this antibody was tested with a series of fresh leukemia cells and leukemia cell lines. Immunoprecipitation of the antigen identified by this antibody demonstrated a 65,000 dalton protein. Another monoclonal antibody, also an IgG1, produced against human monocytes, reacted with human granulocytes, monocytes, eosinophils, and large granular lymphocytes. This antibody reacted with myeloid precursor cells in the bone marrow and reacted with a small proportion (20%) of the acute myelogenous leukemia cells tested. The molecular weight of the antigen identified by this antibody was 95,000 daltons. A series of monoclonal antibodies produced against human platelets were isotyped as IgG1, IgM and IgG2a. One of these antibodies appeared to react with platelets and granulocytes while the other two reacted exclusively with platelets and no other circulating normal cells. Further characterization of these antibodies, including the molecular characterization of the antigens they react with and their effect on platelet function, is ongoing. An IgG1 monoclonal antibody has been produced against human eosinophils. The antibody also reacted with monocytes, granulocytes, and myeloid precursor cells in the bone marrow. This antibody immunoprecipitated a 95,000 dalton protein.